It is well known that the higher the viscosity of a liquid composition, the greater will be its residence time when applied to a non-horizontal surface such as a wall. Viscosity can be increased in many ways e.g. by the use of a polymeric organic thickening agent as a component of the composition, by increasing the concentration of dissolved components, by adding solid components which are suspended in the solution or by modifying the characteristics of the dissolved components to create gel phases.
Each of these approaches has its limitations. A polymeric thickening agent, although of value in compositions that are not exposed to aggressive aqueous environments, is not useful where the composition contains a hypochlorite bleach because of the tendency of the hypochlorite to attack the polymer, which leads to the destruction of the latter's thickening capability. Mere increases in the solution concentration of components have a limited effect on solution viscosity and are thus not particularly cost effective. The addition of solid, i.e. non-soluble, components introduces additional complexity, in that settling out or sedimentation on storage has to be avoided, and the physical form of the product is normally limited to an opaque suspension which is not ideal for an aqueous cleaning composition. Modification of the physical characteristics of the dissolved components by interaction to form viscous phases can also introduce limitations on the type and concentration of the components.
In order to overcome the problem of thickener (and bleach) stability in thickened aqueous hypochlorite-containing compositions, a variety of formulations have been proposed. Most of these involve combinations of surfactants that are stable to hypochlorite solution, examples being the compositions disclosed in BP No. 1329086 and BP 1418671, European Published Patent Applications Nos. 21581 and 30401 and French Pat. No. 2355909. Hypochlorite bleach compositions containing surfactant combinations with product viscosity values of up to .apprxeq.150 mPa.sec are disclosed by the art but the attainment of higher viscosities than this is not specifically taught and is believed to require surfactant levels that are likely to be unattractive economically.
It has been found by the Applicants that shear thinning behaviour is a desirable characteristic for thickened aqueous hypochlorite-containing compositions intended for use on non horizontal ceramic surfaces. Sheat thinning allows the development of very high viscosities at the low rates of shear which are produced as a result of the movement of a liquid down a vertical surface under its own weight, whilst giving rise to low viscosities when the solution is dispensed under pressure through a restricted orifice such as the neck of a flexibility sided bottle. The commonly assigned Stoddart U.S. Pat. No. 4,576,728 discloses compositions displaying this characteristic which compositions comprise aqueous solutions of long chain amine oxides in combination with certain aromatic compounds having a carboxylic or hydroxylic functionality and possessing a defined amphiphilic character. However, it has also been found that thickened liquid compositions incorporating low levels of one or more additives to produce shear thinning, also show a tendency towards viscoelastic behaviour, particularly at temperatures in the range 5.degree.-20.degree. C. commonly encountered in e.g. toilet bowls. This is a less desirable characteristic as it results in uneven distribution of the liquid over the treated surface. It is also less attractive aesthetically to the consumer. Accordingly there is a need for a thickened aqueous cleaning composition displaying shear thinning characteristics whilst exhibiting controlled, or more prefereably substantially no viscoelastic behaviour.